1. Field of the Invention
The present invention relates to a detecting circuit to be incorporated in an integrated circuit device for checking the connected condition of an input signal line to the integrated circuit device.
2. Description of the Prior Art
The integrated circuit device has a semiconductor chip with an electrical circuit formed integrally therein, a package housing the semiconductor chip, and external leads attached to the package and electrically connected within the package to electrodes of the semiconductor chip. In practical application, the external leads of the integrated circuit device are inserted into terminals of an IC socket which is in turn attached to a printed circuit board. An electronic system usually employs a number of the integrated circuit devices. If happens sometimes, however, that the contact between one or more terminals of the IC socket and the associated external leads of the integrated circuit device is incomplete or disconnected. This causes the disconnection of a signal line and obstructs a predetermined operation of the electronic system even if its circuit construction and all the integrated circuit devices employed therein are normal. This disconnection of the signal line is remarkably difficult to visually discover and may be difficult to discover by an operational analysis. It is required that the connected condition of the signal line can be easily as the whole system becomes more complicated. While the detection of the disconnection in an output signal line of an integrated circuit device is relatively easy because the connection of the output signal line can be detected by checking the output signal on a printed circuit board, the detection of the input signal line disconnection is very difficult. This detection of the disconnection of the input lead of an integrated circuit is more important than the output lead disconnection, because the connected condition of the output lead can be easily detected by checking the output signal.
A circuit for detecting the disconnection on the input side of an integrated circuit has been proposed in U.S. Pat. No. 4,183,460 granted to Raymond C. Yuen. According to this detection circuit, an input signal is supplied from an input lead of an integrated circuit device via an input electrode on a semiconductor chip to the base of an emitter-follower transistor formed in the semiconductor chip which has its emitter fed with a constant current, and a detecting transistor is also formed in the chip to construct a differential amplifier together with the emitter-follower transistor. A reference voltage lower than not only the high level but also the low level of the input signal is supplied to the base of the detecting transistor. The emitter of the detecting transistor is connected with the emitter of the emitter-follower transistor. The input signal fed to the base of the emitter follower transistor is led out of the emitter thereof and applied to an input of an integrated circuit formed on the same chip.
According to the detecting circuit in the prior art, when an input lead of the integrated circuit device is disconnected to a terminal of an IC socket, the detecting transistor is rendered conductive, so that the connected condition of the input signal line can be detected by checking the collector voltage of that detecting transistor. Since the detecting transistor is connected to the emitter of the emitter-follower transistor, however, there arises a parasitic capacitor based upon the detecting transistor at the emitter of the emitter-follower transistor from which the input signal is derived. This results in a long transmission time of the input signal so that the applicable frequency limit of the input signal is lowered. Another disadvantage is that one detecting transistor has to be added with respect to each of the input electrodes of the semiconductor chip so that the number of the additional transistors is inevitably increased in a large-scale integrated circuit device having a number of input leads.